A priori knowledge on large-scale sub-surface conductivity structure is required in many applications investigating electrical properties of the lithosphere. A map on crustal conductivity for the Fennoscandian Shield and its surrounding oceans, sea basins and continental areas is presented. The map is based on a new database on crustal conductance, i.e. depth integrated conductivity, where all available information on the conductivity of the bedrock, sedimentary cover and seawater are compiled together for the first time for the Fennoscandian Shield. The final model consists of eight separate layers to allow a 3D description of conductivity structures. The first three layers, viz. water, sediments and the first bedrock layer, describe the combined conductance of the uppermost 10 km. The other five bedrock layers contain the data of the crustal conductance from the depth of 10 km to the depth of 60 km. The database covers an area from 0• E to 50• E and 50• N to 85• N. Water conductances are estimated from bathymetric data by converting depths to conductances and taking into account the salinity variations in the Baltic Sea. Conductance of the sedimentary cover includes estimates on the conductance of both marine and continental sediments. Bedrock conductances are extrapolated from 1D-and 2D-models. Extrapolations are based on data from magnetometer array studies, airborne electromagnetic surveys and other electromagnetic investigations as well as on other geophysical and geological data. The crustal conductivity structure appears to be very heterogeneous. Upper crust, in particular, has a very complex structure reflecting a complex geological history. Lower crust seems to be slightly more homogeneous although large regional contrasts are found in both the Archaean and Palaeoproterozoic areas.

We have compiled a global three-dimensional (3D) conductivity model of the Earth with an ultimate goal to be used for realistic simulation of geomagnetically induced currents (GIC), posing a potential threat to man-made electric systems. Bearing in mind the intrinsic frequency range of the most intense disturbances (magnetospheric substorms) with typical periods ranging from a few minutes to a few hours, the compiled 3D model represents the structure in depth range of 0-100 km, including seawater, sediments, earth crust, and partly the lithosphere/asthenosphere. More explicitly, the model consists of a series of spherical layers, whose vertical and lateral boundaries are established based on available data. To compile a model, global maps of bathymetry, sediment thickness, and upper and lower crust thicknesses as well as lithosphere thickness are utilized. All maps are re-interpolated on a common grid of 0.25 × 0.25 degree lateral spacing. Once the geometry of different structures is specified, each element of the structure is assigned either a certain conductivity value or conductivity versus depth distribution, according to available laboratory data and conversion laws. A numerical formalism developed for compilation of the model, allows for its further refinement by incorporation of regional 3D conductivity distributions inferred from the real electromagnetic data. So far we included into our model four regional conductivity models, available from recent publications, namely, surface conductance model of Russia, and 3D conductivity models of Fennoscandia, Australia, and northwest of the United States.

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A large‐scale international electromagnetic experiment has been carried out in northwest Poland and northeast Germany. The main goal was to study the deep conductivity structure across the Trans‐European Suture Zone, which is the most prominent tectonic structure of Phanerozoic age in Europe. Electromagnetic measurements were carried out mainly along seismic profiles P2, LT‐7, and LT‐2 crossing the suture zone and running in the northeastern direction. Strike and dimensionality analyses indicate that a geo‐electrical strike of N60°W common to both profiles LT‐7 and P2 can be estimated. This strike direction was used to project and rotate all transfer functions and both profiles were subjected to 2D inversion using three different approaches. The results show the presence of highly conductive Cenozoic‐Mesozoic sedimentary cover reaching depths up to 3 km. A significant conductivity anomaly beneath the central part of the TESZ, called the Central Polish Anticlinorium, has been well resolved at mid‐crustal depths. The upper mantle of the Precambrian East European Craton is more resistive than, adjacent to the West, the younger Paleozoic Platform.

S U M M A R YGeomagnetic induction responses such as geomagnetic depth sounding (GDS), magnetotelluric (MT), and horizontal geomagnetic transfer function (HTF) at long periods are used to estimate the electrical conductivity in the deep mantle. The responses in the period range that are shorter than 10 5 s (about 1 day) are in many cases considered to be local or regional induction problems in which the source field is approximated by plane waves and therefore the sphericity of Earth is not taken into account. In the period range between 10 4 and 10 5 s, the most dominant signature of the magnetic field variation is the solar quiet daily (Sq) variation and its higher harmonics. Therefore, when we obtain the responses due to the quasi-white background spectrum composed of plane waves, we regard the Sq field variations as noises in estimating the responses, and line spectra of the variations are removed from the observed time-series before the responses are calculated. However, with this approach, the calculated responses tend to possess a discontinuity at a period of about 10 4 s, and the response functions show common features at longer periods irrespective of the location of the observation site. It is particularly well known that the imaginary part of the induction vector tends to have a significant westward component for periods ranging between 10 4 and 10 5 s. Such features cannot be easily explained by the effect of the electrical conductivity structure alone. Examination of the phase of the HTF implies that the responses in the same period range are affected by the signature of sources of finite wavelength moving westward. Thus, it is suggested that the response functions in this period range were under the effect of the Sq field variations, even though the line spectra of them were removed and the responses were estimated at periods separate from the harmonic periods of Sq field variation. We examined how the influences of the Sq field appear on the responses numerically by a forward modelling. Results show most of the characteristic features in observed response functions can be ascribed to Sq source effects.